1. Field of the Invention
This invention relates to an audio signal recording apparatus and more particularly to an apparatus which records an audio signal as a digital signal.
2. Description of the Related Art
A digital recording apparatus which records video and audio signals as digital signals on a magnetic tape with a rotary head in a helical scanning manner is known by the name of a digital VTR. In the case of an analog VTR which records video and audio signals as analog signals on a magnetic tape in the helical scanning manner, the VTR is arranged to be operable either in a standard-time recording/reproducing mode in which recording and reproduction can be performed for a standard length of time (hereinafter referred to as the SP mode) or in a long-time recording/reproduction mode in which recording and reproduction can be performed over a longer period of time (hereinafter referred to as the LP mode). The digital VTR of the above-stated kind is also arranged to permit recording and reproduction in the LP mode by compressing the recordable amount of data per unit time.
FIG. 1 is a block diagram showing in outline the arrangement of the conventional digital VTR. An input terminal 10 is arranged to receive the luminance signal component Y of a video signal. Input terminals 12 and 14 are arranged to receive the chrominance signal components PR and PB of the video signal. Input terminals 16, 18, 20 and 22 are arranged to receive audio signals of four channels. The analog signals supplied to these input terminals 10 to 22 are digitized by A/D converters 24, 26, 28, 30, 32, 34 and 36. The digital video signal data which is thus obtained is temporarily stored by a memory 38V. The digital audio signal data thus obtained is temporarily stored by a memory 38A. A video encoder 40 is arranged to perform a recording processing action on the video data stored in the memory 38V before recording it on a recording medium. An audio encoder 42 is arranged to perform a recording processing action on the audio data stored in the memory 38A also for recording on the recording medium. An adder 44 is arranged to add together (time-base multiplex) the output of the video encoder 40 and that of the audio encoder 42 and to output them in a time-base multiplexed state. A recording amplifier 46 is arranged to amplify the output of the adder 44. A rotary head 48 is arranged to record the output of the recording amplifier 46 on a magnetic tape 50 by electro-magnetically converting the output of the amplifier 46.
However, if recording is performed in the LP mode capable of recording for a period of time which is, for example, twice as long as the time of the SP mode, the above-stated arrangement presents a problem which is as described below:
In the case of an analog VTR, the amount of information of the analog audio signal to be recorded in each track changes little in the LP mode, since it is only the track pitch that is reduced to one half while the number of tracks formed per unit time remains unchanged. Further, even in the event of an analog VTR which is arranged to record a digital audio signal along with an analog video signal, the amount of information of the audio signal does not change at all.
Whereas, in the case of the digital VTR, the number of tracks formed per unit time becomes 1/2 of it. Therefore, the amount of information of the audio signal to be recorded per unit time also must be reduced accordingly. However, a mere arrangement to perform high-efficiency encoding by utilizing the correlation of signals in reducing the amount of data of the audio signal of each channel to 1/2 of it greatly degrades the quality of the audio signal, because: unlike the video signal, the audio signal does not have much correlation between adjacent samples. Besides, for the audio signal, it is impossible to use any two-dimensional correlation (obtained in the vertical direction of an image) or three-dimensional correlation (in the direction of time).